Masts of storage and retrieval machines are subject to impact loads if the machine runs out of control or objects strike the mast, or if the carriage movably mounted on the mast hits an object during either horizontal or vertical movement of the carriage. Thus, the structural design of the mast needs to provide sufficient strength to resist the high stresses encountered in the mast during an impact condition. The stresses in the mast are particularly high as a result of high inertial forces resulting from deceleration of the mast when the crane runs out of control and impacts a buffer. The resulting moment force increases in magnitude toward the lower end of the mast and therefore high structural strength of the mast at its lower end is particularly important. The stresses in the mast are contributed to by the mass of the mast itself and also by the mass of the carriage supported on the mast. The higher the position of the carriage on the mast at the time of impact to the mast, the greater the stresses will be on the mast.
In addition to supporting the carriage, the mast must incorporate guide means against which the wheels of the carriage roll as it moves vertically on the mast. Due to the weight of the carriage plus the load that it carries, the strength of the guide means must be fairly substantial and this adds further to the problems in designing a mast of adequate structural strength.